1. Field of the Invention
This invention relates to improving the wear resistant properties of metal parts and in particular, the erosion resistance of those parts due to sand rock and other debris entrained in fluids passing across the surfaces of those parts. This invention has particular application to improving the surface properties of bearings and rotors such as are used in down hole motors or alternatively in turbines.
2. Description of Related Art
The application of hard metal particles to the surfaces of metal parts is a well known method for improving the wear resistance of such parts. U.S. Pat. No. 3,936,295 issued to Cromwell et al. and entitled "Bearing Members Having Coated Wear Surfaces" discloses bearing members having a coated wear surface applied by plasma-spraying an aggregate of particles onto the surface of the bearing members. More specifically, Cromwell et al. discloses that these particles consist of an aggregate of nickel-aluminum, nickel-molybdenum, tungsten carbide and an intermetallic alloy which are alloyed together during plasma spraying onto the wear surface. The wear surfaces that were coated in Cromwell et al. were the surfaces of a piston in an internal combustion engine. The wear resistant surface achieved through this particular combination of particles was intended to maintain the bearing seal between the piston and cylinder wall without inducing wear through abrasion of the cylinder wall and without sacrificing mechanical strength.
The particles in Cromwell et al. were applied using conventional flame-spray techniques using a plasma arc gun. Much of the art concerning the application of hard metal particles to substrates relies on the use of plasma arc processes and an outline of such procedures is believed to be appropriate. Cromwell et al. describes that "The plasma flame of such a gun (plasma arc) produces temperatures of approximately 32,000 degrees Fahrenheit achieved by applying electrical energy to a gas mixture (such as, for example, a ten to one nitrogen to hydrogen mixture) which causes the gas molecules of the mixture to dissociate into an atomic state. The gases are then ionized, producing electrons and charged ions. The electrical energy absorbed by such ionization is converted to heat energy by de-ionization of the gas. The aggregate is directed into the plasma flame by a carrier gas such as, for example, nitrogen. The particles of the aggregate are propelled by the gas escaping through the nozzle of the gun as a stream of molten particles. The nozzle is aimed at the surface to be coated so that the molten particles impinge thereon. The molten particles solidify to provide a continuous, adherent coating on the surface that results from a combination of mechanical and atomic bonding at the interface of the coating and the substrate body and between the particles themselves to form an alloy of the constituents of the starting aggregate."
U.S. Pat. No. 5,346,316 issued to Okada et al. describes that spray coatings generally contain hard particles such a tungsten and chromium carbides bound with molten metals such as nickel, chromium and cobalt. It is further explained that the surface of an article having such a coating is lower in hardness than the same article made of solid sintered tungsten or chromium carbide. The differences in hardness are reportedly due to defects in the binder metals such as blow holes and/or insufficient binding strength between the hard metal and the binder metal. Okada et al. claims to overcome the deficiencies of spray coating by subjecting the coated article to a heat treatment of 300 to 500 degrees Celsius for a period of not less than one hour. The bearing that is described and claimed in Okada et al. is intended for use in a drainage pump and reportedly exhibits improved wear resistance against water containing earth and sand.
The use of bearings in down hole motors in drilling operations is well known. U.S. Pat. No. 4,329,127 issued to Tschirky et al. and entitled "Sealed Bearing Means For In Hole Motors" generally describes the environment in which these bearings are used. It specifically states that in the drilling of bore holes into or through earth, as in the case of drilling oil and/or gas wells or in certain mining or other earth boring operations, a practice has been to drive the drill bit by a fluid motor installed in a drill pipe string and through which drilling fluid is circulated to drive the fluid motor and then pass through the bit nozzles into the drill hole to flush away cuttings. The drilling fluid and entrained cuttings are then returned to the drilling rig or to the surface through an annulus outside drill string and outside the motor. The drill string applies weight to the bit with the weight of the string being transferred through a bearing assembly which rotatably supports a hollow drive shaft within an elongated housing. The drive shaft is driven by the rotor of the fluid motor, while the bearing housing is fixed to the drill pipe string and remains relatively stationary. The bearing means between the drive shaft and the housing must sustain severe vibration, shock, axial and radial loading.
The bearing assemblies disclosed in the prior art are generally either a sealed assembly in which the drilling mud and entrained debris are prevented from contacting the bearing surfaces or unsealed bearing assemblies wherein the circulating drilling fluids pass directly through the bearing assembly. Tschirky et al. '127 is an example of a sealed bearing assembly in which the bearing elements are protected within a sealed chamber that maintains the bearings in a lubricant fluid. Although the sealed bearing assemblies prevent drilling debris and other corrosive elements in the drilling fluid from contacting and eroding the bearing surfaces, it has been found that there is a tendency for the seals of the bearing chamber to fail in this high pressure and corrosive environment. Further, the use of sealed bearings is accompanied by other problems that interfere with the operation of the hydraulic motor.
U.S. Pat. No. 4,029,368 also issued to Tschirky et al. is an example of an unsealed bearing in which the drilling fluids and entrained debris are at least in part passed directly through the bearing assembly. The passage of the drilling fluids and cutting debris through the bearing assembly can cause a significant amount of abrasion to and erosion of the bearing surfaces in contact with those fluids. The wear resulting from this erosion contributes to the rapid failure of the bearing.
Tschirky et al. '368 discloses a radial bearing made of hard, rigid metal in order to overcome the problems of the rubber radial bearings that had previously been used in the prior art. In particular, Tschirky et al. '368 describes the use of tungsten carbide or some other metal that is harder than the sand and other debris that may be entrained in the drilling fluid. Specifically, this bearing includes a tungsten carbide sleeve that is fixed to the stationary portion of the bearing. The rotating member of the bearing is a steel sleeve that is grooved along its length to receive a plurality of circumabiently spaced tungsten carbide rod inserts. The fluid passageways in the bearing are between the stationary sleeve and the rotating steel sleeve and tungsten carbide rod inserts.
It is explained that the rod inserts were previously made of solid pieces of hard metals that tended to be brittle and break apart or spall when subjected to transverse impacts. Spalling is the failure of a hard brittle material during high point loading in compression. To overcome the brittleness and the potential failure of the inserts, the tungsten rods and sleeve of Tschirky et al. '368 are made by dispersing hard metal particles in a metal matrix powder that is then placed in a mold and heated to a temperature that fuses the metal and bonds the hard particles. The metal matrix produced by this process reportedly overcomes the brittleness problem. However, the disclosure does not address the problem wherein the hard metal particles of the metal matrix cause abrasion to the surfaces of adjacent parts.
U.S. Pat. No. 4,720,199, issued to Geczy et al. and entitled "Bearing Structure For Downhole Motors" is an example of another bearing that is composed of hard metal matrix that is molded into a desired shape. As explained, the bearing assembly of down hole motors will typically have two types of bearing members, one to accommodate radial loads and one to accommodate thrust loads. The radial bearing surface of Geczy et al. '199 comprises a macro crystalline tungsten carbide powder blended together with cemented tungsten carbide cobalt chips. The bearing is made by placing a steel bearing within a cavity, surrounding the steel bearing with the blended mixture and infiltrating the mixture with copper. The copper infiltrant is loaded on top of the blended mixture and infiltrates down into the mixture as the mold cavity is heated.
U.S. Pat. No. 4,732,491, also issued to Geczy et al. and entitled "Downhole Motor Bearing Assembly", describes how the process of Geczy et al. '199 can be used to incorporate the radial and thrust bearing elements of the bearing assembly directly into the drive shaft of the hydraulic downhole motor. Specifically, it is explained that a radial bearing surface may be applied to the cylindrical interior of the stationary housing and the cylindrical exterior surface of the rotating drive shaft, and that mounting sites may be provided on the drive shaft for inserting thrust bearing inserts opposite the thrust inserts on the housing.
U.S. Pat. No, 4,277,108 issued to Wallace and entitled "Hard Surfaced Well Tool And Method Of Making Same" describes a method for placing a wear resistant surface on tool joints in a drilling string. In particular, it cites that previous methods had involved placing a thin layer of hard banding material, namely, tungsten carbide particles, on tool joints. It is also explained how a layer of mild steel on top of the layers of sintered tungsten carbide particles further reduces the wear of the tool joint surfaces and provides abrasive protection to the well casing. According to Wallace, a wear resistant tool joint is preferably made by laying down multiple layers of first larger and then smaller sintered tungsten particles in a steel matrix. Optionally, a layer of mild steel may be laid down over the multiple layers of hard metal particles. However, the disclosure of Wallace is specific to tool joints and it does not suggest or describe a technique for improving the wear resistance of the surfaces of bearings or other metal parts that are exposed to highly erosive environments.
None of the previously discussed prior art methods or devices have a bearing surface that is sufficiently wear resistant for long term use in a highly erosive environment. Attempts have been made to improve wear resistance by increasing the thickness of the hard metal particle layer that is applied by spray coating or molding. However, it has been found that layers of hard metal particles that have a thickness of greater than 3/16 of an inch will suffer considerable spalling failure. Therefore, there is still a need for a low cost bearing member or other metal parts with hard surfaces that do not easily erode or wear.
It has also been found that although the application of hard metal particles to the surface of one element improves the hardness and wear resistance features of that element, the tendency is for those hard metal particles to act as an abrasive on the surfaces of adjacent elements. As such, the improvements in one part may be at the expense of another. Therefore, there also remains a need for a wear resistant surface and surfacing process which improves wear resistance in the subject part but does not induce wear on adjacent parts.